This invention relates to the glossing of thermoplastic resins on microencapsulated receiver sheets as disclosed in the commonly assigned U.S. Pat. Nos. 4,399,209 and 4,416,966.
The art of glossing polymer resins on substrate sheets is discussed in Cowgill, U.S. Pat. No. 2,554,663 issued May 29, 1951. Cowgill described the glossing of such resins in terms of the second order transition temperature of the resin (T.sub.2), also known as the glass transition temperature (T.sub.9). The second order transition temperature as defined by Cowgill is that temperature at which the amorphous thermoplastic resin material changes from a two dimensional liquid to a three dimensional liquid. At this point the resin is said to begin to flow more readily as the temperature is increased. As the temperature is increased, the viscosity goes down and the tackiness of the material increases.
Cowgill believed that the setting of the coating by heating to a temperature well above the second order transition temperature was essential in order to obtain satisfactory glossing. Cowgill also showed that the addition of a plasticizer could be used to lower the spread between the second order transition temperature T.sub.2 and the actual glossing temperature, and suggested an oven for preheating the resin to a given temperature above T.sub.2, and a roll-type casting apparatus including a heated mirror finish roll against which the web is pressed face down and wrapped about 180.degree. and then peeled off face up and cooled. The mirror finish roll was maintained at a temperature above the glass transition temperature, by an actual amount dependent upon the amount of plasticizer employed.
Cowgill's investigation also showed that it was possible to reduce the glossing temperature with increased pressure at the nip of the pressure rolls, but that the decrease in glossing temperature as the pressure was increased from 200 to 1,500 psi only amounted to approximately 10.degree. C., where the s, ting temperature was already 40.degree. C. above T.sub.2.
With respect to the resin-coated receiver sheets of the kinds described in the above-identified commonly owned patents, it has been found that coalescing resin coating improves the perceived image quality by increasing color saturation. While Cowgill was primarily interested in coating paper stock such as may be used in the manufacture of cartons and the like, and employed primarily a resin copolymer of styrene and butadiene, current technology prefers the use of vinyl polymers and copolymers as disclosed in commonly assigned copending application Ser. No. 086,059 filed Aug. 14, 1987 now U.S Pat. No. 4,877,767, or phenolic resins as disclosed in commonly assigned copending application Ser. No. 073,036 filed July 14, 1987 now U.S. Pat. No. 4,859,651. Such phenolic resins may be metallated to improve their characteristics, and plasticizers may be employed to lower the glass transition temperature and to improve surface quality.
Such resins as currently used are obtained in a dispersed form and are roll and/or blade coated. Commonly the resin is finely ground or finely divided and dispersed in a carrier oil, and the resulting coating on the substrate, has a rather opaque, white, or milky appearance. In the processing or glossing stage, it is necessary to transparentize this coating and provide thereon a suitable smooth high gloss surface condition.
The milky or opaque appearance of the coating, prior to transparentizing, is believed to be due to the reflection and refract on of light at the uneven surface, and additionally due to the breaking up of the light at the interfaces between the carrier and the dispersed particles. The latter may be viewed as small globules or spheres of particles under a microscope at between 200 and 500 power magnification.
The copolymer has a glass transition temperature T.sub.9 or range in which the coating constituents, each having approximately the same index of refraction, coalesce into a common surface where the individual parts can no longer be visually or optically distinguished, and one which has a minimum of residual haze and a maximum of gloss, as may be measured on a conventional gloss meter.
Substantial efforts have been expended to find glossing apparatus and methods which are compatible with the microencapsulated system, and which can provide satisfactory glossing at a throughput rate compatible with sheet development. At the present time, this rate is considered to be at least 10 ppm (pages per minute) for paper substrate and 5 ppm for overhead projection substrate (based on 81/2.times.11 inch paper). Such investigations have included the evaluation of hot roll as well as heated platen type glossing apparatus, and the commonly assigned U.S. Pat. No. of Stone et al, 4,807,560 issued Feb. 28, 1989 discloses and claims satisfactory developer sheet glossing apparatus employing a heated arched plate and a casting belt.
At the same time, heated roll-type glossing apparatus was investigated, but as stated in U.S. Pat. No. 4,807,560, less than satisfactory results were obtained. A principal drawback was the tendency of the heated resin to adhere to the hot rollers. Such adhesion not only causes misfeeds, but also causes degradation to the image and surface.
In connection with the investigation, it was discovered that the portions of the sheet being glossed having the lowest image density were those portions which required the highest glossing temperature. The reason for this phenomena is not fully understood, but it is believed that the dye precursor and oil released from the ruptured microcapsules may assist as a plasticizer in the film-forming characteristics of the resin. Such dye precursor and/or oil is much more abundant in the dense or darker areas than in the areas of minimum density. Therefore, it has been necessary to adjust the glossing temperature in accordance with the most difficult areas to be glossed, namely, those of minimum density. This is further exacerbated by the sticking problem when attempting to use roll-type glossers. One reason why the heated arched plate glosser of the above-identified patent was successful was due to the fact that the resin surface was formed by the surface of belt and not the plate, and was subsequently peeled off the belt.
The apparatus employed in the tests of the heated roll glosser was published in April 1987 in Article No. 27660 in Research Disclosure, Emsworth Studios Inc., 260 West 39th Street, New York, N.Y. 10018.
High temperature and high pressure roll-type glossing of latex coated pigment and latex coated paper stock is shown in Vreeland, U.S. Pat. No. 3,873,345 issued Mar. 25, 1975 and Vreeland divisional Pat. No. 4,112,192 issued Sept. 5, 1978. These prior art disclosures also describe the glass transition temperature phenomena T.sub.9 and employ a hot steel roll in the gloss calendar which is maintained at a temperature in excess of T.sub.9, and at a substantial nip pressure such as 400 pli (lbs. per linear inch). Since mineral pigmented coatings were employed, the transparentizing of the coating was not a necessary objective, and the results were measured in terms of surface gloss. Vreeland, U.S. Pat. No. 4,624,744 issued Nov. 25, 1986 further discloses the calendering of uncoated papers under heat and pressure to provide a surface gloss, using a drum surface in excess of the glass transition temperature of the uncoated cellulose paper fibers.
Very smooth and high gloss skins of silicone rubber have been applied to rolls for the calendering of synthetic resin materials, and the rolls have been manufactured as disclosed in Nauta et al, U.S. Pat. No. 4,368,240 issued Jan. 11, 1983. The rolls as made in this reference were used in the smoothing and calendering of webs formed of synthetic thermoplastic material.